To convert Celsius to Fahrenheit, multiply by 9, divide by 5, then add 32. To convert Fahrenheit to Celsius, subtract 32, multiply by 5, then divide by 9. So 25°C is 25 × 9 ÷ 5 + 32 = 77°F. The temperature converter does both, and shows Kelvin too.
That is the formula. Here is why it has those two steps, and how to estimate without a calculator.
Why two steps, not one
Temperature is different from length or weight, where you just multiply by a factor. The Celsius and Fahrenheit scales disagree in two ways at once: their degrees are different sizes, and their zero points are in different places. Water freezes at 0°C but 32°F, and boils at 100°C but 212°F. So a conversion has to both rescale the degree (the × 9 ÷ 5 part) and shift the starting point (the + 32 part). That is why it is a formula rather than a single multiplication.
How to convert temperature
Celsius to Fahrenheit
Multiply by 9, divide by 5, add 32.
- 0°C → 32°F
- 37°C → 98.6°F (normal body temperature)
- 100°C → 212°F
Fahrenheit to Celsius
Subtract 32, multiply by 5, divide by 9.
- 32°F → 0°C
- 70°F → 21.1°C
- 212°F → 100°C
The temperature converter handles both directions instantly.
Fast mental shortcuts
For a quick weather estimate, two tricks help:
- Celsius to Fahrenheit: double and add 30. So 20°C is about 70°F (exact 68), and 30°C is about 90°F (exact 86). It drifts a little at the extremes but is fine for deciding what to wear.
- Fahrenheit to Celsius: subtract 30 and halve. So 70°F is about 20°C.
And the fact worth memorising: −40°C and −40°F are the same temperature, the one point where the scales meet.
Where it matters
The full formula matters when precision counts: oven temperatures in a recipe, a fever reading, or scientific work. For those, use the exact conversion rather than the shortcut, since a few degrees can change a result. For Kelvin, used in science, add 273.15 to the Celsius value.
To convert speeds between systems the same way, see the speed converter.